With the continued emphasis on highly integrated electronic devices, there is an ongoing need for semiconductor devices that operate at higher speed and lower power and have increased device density. To achieve these goals, it is necessary for devices to be formed with increased integration and for device components to be formed of lower-resistivity materials. At the same time, there is a need for analog circuitry in modern digital systems. For example, in the peripheral region of a semiconductor memory device, there is a need for resistors to be formed with resistance values that are predicable and within tight tolerance levels. With the continued pressure toward further integration of devices, it is increasingly difficult to fabricate resistors having accurate resistance values.
In contemporary devices, transistor gates are commonly formed of WSix, as this material provides relatively low resistance, and reliable application. At the same time, resistors can also be formed to have somewhat predictable values using WSix.
With increased integration, it is preferred to form the gate material of transistor devices using different silicide materials such as CoSix and NiSix, or metal gate materials such as W, Cu or Al, for example, since such materials can be formed to have lower resistance. However, when using such materials to form a resistor, it has been determined that the resistivity of such materials is highly susceptible to heat and thickness. For example, when a silicide layer is used, the resistivity of a resulting resistor will vary greatly in accordance with succeeding processes that can generate a heat budget, since the properties of CoSix or NiSix can be varied with a varying heat budget. Also, when a metal layer is used to form the resistor, the metal layer thickness can be varied widely by an applied chemical-mechanical polishing (CMP) procedure that is used during fabrication of the metal gates of the device. This in turn can cause so-called “dishing” of the resistor material, which can greatly change the resistance value of the resulting resistor.